Support custom names for registered functions

[simonw]

In this example:

    @db.register_function
    def reverse_string(s):
        return "".join(reversed(list(s)))

    print(db.execute('select reverse_string("hello")').fetchone()[0])

There's currently no way to over-ride the automatically selected name for the SQL function.

[simonw]

Updated documentation: https://github.com/simonw/sqlite-utils/blob/1491b66dd7439dd87cd5cd4c4684f46eb3c5751b/docs/python-api.rst#registering-custom-sql-functions